Polymeric membrane and process for separating aliphatically unsaturated hydrocarbons

ABSTRACT

A membrane is provided for separating aliphatically unsaturated hydrocarbons from hydrocarbon mixtures, the membrane comprising a hydrophilic polymer which contains metals capable of complexing with aliphatically unsaturated hydrocarbons.

This a continuation of application Ser. No. 256,666, filed Oct. 31,1988, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to the separation of aliphaticallyunsaturated hydrocarbons from hydrocarbon mixtures. More particularly,the present invention relates to membranes suitable for the separationof olefins from paraffins.

The aliphatically unsaturated hydrocarbons, which typically are obtainedin admixture with other hydrocarbons as the by-products of chemicalsyntheses or separations, are important reactive materials for preparingpolymers and in other applications. While distillation of theunsaturated hydrocarbons from the streams in which they are found isfeasible when the hydrocarbons are normally liquid or can readily bemade so and the boiling points of the other feedstream components differsufficiently, more expensive procedures, such as cryogenic distillationor extractive distillation, are required when the feedstream is gaseousat ambient conditions or the components of the feedstream areclose-boiling. Thus there is considerable commercial motivation fordeveloping alternative processes for separatingaliphatically-unsaturated hydrocarbons from the hydrocarbon streams inwhich they are found.

The known property of aliphatically unsaturated hydrocarbons to complexreversibly with certain metals or metal ions, particularly transitionmetals such as silver and the salts thereof (see, e.g., Chatt, J.,"Cationic Polymerization and Related Complexes --The General Chemistryof Olefin Complexes with Metallic Salts, pp. 40-56, Proc. Conf. Univ.Coll. North Saffordshire, England, 1952), has been the basis for variousprocesses for purifying unsaturated hydrocarbons.

U.S. Pat. No. 2,685,607, for example, proposes separating olefins fromparaffins by contacting with silica gel impregnated with aqueous silvernitrate. U.S. Pat. No. 2,458,067 employs a solution of silver inacetonitrile as an extractant. See also U.S. Pat. No. 3,395,192. U.S.Pat. No. 4,174,353 discloses a process for separating ethylene orpropylene from a hydrocarbon cracking stream by extracting the olefinsinto an aqueous silver salt solution.

Reversible complexing agents have been employed in association with amembrane support for "facilitated transport" of unsaturated hydrocarbonsthrough the membrane to achieve purification or separation of theunsaturated hydrocarbons from mixtures thereof. See, for example, WardIII et al., Science, 156, 1481 (1967); Way et al., J. Mem. Sci., 12, 239(1982); Way et al., AIChE J., 33, 480 (1987) and Way et al., SRIInternational, Research Brief (May 15, 1987).

The metal complexing agent is selected so that the complex of metal andaliphatically unsaturated hydrocarbon forms readily and also readilyreverts to its separate constituents under the conditions which exist onthe permeate side of the membrane. The released unsaturated hydrocarbonshaving permeated the membrane are removed from its vicinity by suitablemeans such as by a sweep gas or through the effect of vacuum. While inthe absence of the complexing metal there may occur some slightseparation of feed components due to differing permeabilities across themembrane support, the presence of the metal complexes in associationwith the membrane provides enhanced selectivity for the unsaturatedhydrocarbons.

U.S. Pat. No. 3,773,844 to Perry (assigned to Monsanto Company)discloses a membrane pervaporation process for separating mono-alkenesfrom hydrocarbon mixtures. The polymeric membrane contains a transitionmetal such as silver molecularly dispersed therein, the metal preferablybeing in an oxidation state to permit chemical interaction between itand the monoalkene, and also preferably chemically interacting with thepolymer material in which it is dispersed, to minimize metal loss.Suitable polymers are indicated to include polyacrylonitrile, polyvinylalcohol, polyvinylchloride, cellulose, cellulose esters, nylon,polyethylene, polystyrene, neoprene, copolymers of acrylonitrile andstyrene, and copolymers of acrylonitrile and other polymers. Preferredpolymers are those which contain groups capable of forming covalent orionic bonds with the metal: for covalent bonding, groups such as theamine, amide, nitrile, alcohol, carbonyl, ether, sulfur, or carbongroups (including groups which contain the carbon-carbon double bond)may be used; for ionic bonding, carboxylate, sulfonic, phosphonate,phosphonic, arsenic and telluric moieties or end groups are employed.

The membranes are formed by casting from a solution or dispersion of thepolymer and a soluble form of the metal, or by melt pressing a mixtureof the powdered polymer and metal.

The '844 patent further teaches that improved monoalkene permeation maybe obtained by a "conditioning"of the membrane prior to use to effectthe replacement of undesirable ligands (e.g. from the solvent) from themetal by ligands which are said to be more easily displaced duringpermeation, thus permitting greater interaction between the metal andthe alkene. This "preconditioning" step comprises soaking the membranein a solution containing the displacing ligand or by casting the polymermembrane from a solution which contains, in addition to the polymer andthe metal species and solvent, an organic material which contains analkene linkage (col. 6, 11. 22-32).

In the Examples of the '844 patent, mixtures of styrene andethylbenzene, and hexene and hexane, are contacted under pervaporationconditions against various polymer membranes such as polyvinylchloride,acrylonitrile polymers or copolymers, an aromatic hydrazide-amidepolymer and an ethylene/acrylic acid copolymer. It is not indicatedwhether any of the membranes was treated by preconditioning.

A separation factor of up to 7.22 was reported for the separation ofhexene from hexane employing a membrane comprising a copolymer ofacrylonitrile and vinylpyridine.

U.S. Pat. Nos. 3,758,603 to Steigelmann et al. and 3,758,605 to Hugheset al. (both assigned to Standard Oil Company) disclose a process forseparating unsaturated hydrocarbons from gaseous mixtures employingliquid barrier permeation and metal complexing techniques.

A liquid barrier comprising an aqueous solution of a metal whichreversibly complexes with the unsaturated hydrocarbon is placed incontact with a semi-permeable membrane which is permeable to thegas-phase hydrocarbon mixture.

The membrane serves to immobilize the liquid barrier adjacent to orwithin the feed side of the membrane. While in the absence of theimmobilized liquid, essentially all of the gas-phase components of thefeedstock may permeate the membrane, the physical passage of the vaporsin the presence of such a barrier is reduced or prevented; and thereforein order to traverse the film, a component of the feed stream mustbecome a part of and then separate from the liquid barrier phase. It isintended that there be little, if any, passage of the feed componentsacross the membrane except by interaction with the liquid barrier, andthus the liquid barrier controls the selectivity of the liquid barriersemi-permeable membrane.

In Steigelmann et al. the membrane is said to be essentially impermeableto the liquid barrier which is placed in contact with it. Suitablemembranes are indicated to comprise cellulose acetate, nylon, polyvinylchloride, polyvinyl alcohols, olefin polymers such as polyethylene,polypropylene and ethylene-propylene copolymers.

In Hughes et al. wherein the liquid barrier is placed within ahydrophilic, semi-permeable membrane, suitable membranes are exemplifiedby the polyurethanes, such as are obtained by reaction ofpolyisocyanates with an aliphatic polyol such as polyvinyl alcohol,although other polymers may be used if made sufficiently hydrophilic byincorporation into the polymer of hygroscopic agents, such as polyvinylalcohols, polyacrylic acids, polyvinyl ethers, polyxyalkylene glycolsand their carboxylic acid esters, and like polymers; as well asnon-polymeric hygroscopic agents such as ethylene glycol, glycerol andpropylene glycol, and alkylated carboxycellulose derivatives.

See also related Standard Oil patents 3,864,418, 3,865,890, 3,940,469,3,951,621, 3,980,605, 4,014,665, 4,060,566, 4,200,714, 4,235,983,4,239,506.

U.S. Pat. No. 4,318,714 to Kimura et al. (General Electric Company)discloses a process for facilitated transport of gases by means of ahumidified ion-exchange membrane containing mobile counterions withinits pores which are said to be retained within the membrane surfaces bythe requirement of maintaining electroneutrality. The membrane is saidto actively take part in the facilitation of gas permeation rather thanserving merely as a support for the immobilized liquid containedtherein.

In the examples, the membrane was prepared by soaking an ion-exchangemembrane in an aqueous solution containing the ion. In Example 4 theseparation of olefins from a gas mixture is reported, using a sulfonatedpolyxylene oxide ion-exchange membrane cast from a solution ofchloroform and methanol, prior to immersion in silver nitrate solutionfor conversion to the silver counter-ion form. The feed gases comprisedeither pure ethylene or pure ethane humidified to 90 percent relativehumidity. Ethylene permeability at 25° C. was indicated to be 230 ×10⁻ 9cc cm/sec cm.sup. 2 cm Hg, and the permeability of ethane undercorresponding conditions was indicated to be 0.8 ×10⁻ 9 cc cm/seccm.sup. 2 cm Hg. An ethylene/ethane separation factor of about 300 wasreported.

U.S. Pat. No. 4,614,524 to Kraus (Monsanto Company) describes awater-free immobilized liquid membrane for facilitated transport ofaliphatically-unsaturated hydrocarbons. The membranes are hydrophilic,semi-permeable preformed polymeric membranes capable of chemicallybonding positive metal ions, and being plasticized by treatment withpolyhydric alcohols. The plasticization with polyhydric alcohols is saidto obviate the need for water in the membrane itself or in the feedstream. Selectivities with respect to a dry ethylene/ethane mixture werereported as about 8 to 15 and permeabilities were from about 5 to 10×10¹⁰ in water-free feedstreams under ambient conditions. Preformedmembranes indicated to be suitable are halogenated polyolefins withpendant acid groups, sulfonated polymers, carboxylated polymers,polyacrylic acids, and the like.

See also Published UK Patent Application GB 2,169,301 of Kraus (alsoassigned to Monsanto) which discloses water-free facilitated transportmembranes which comprise a separation barrier of metal ions in solutionwith one or more polyhydric alcohols, the barrier separation membranebeing incorporated into the pores or on the surface of hydrophobilic orhydrophilic membrane materials.

However, commercial application of immobilized liquid membranes ishampered by such factors as the gradual leaching of the metal-containingsolution from the membrane and the fragility of liquid membranes in thepresence of a transmembrane pressure difference. Further, efforts toachieve satisfactory flux in separations processes employing suchmembranes have been hindered by the constraints on membrane thinnessimposed by the necessity to provide adequate support for the liquidphase.

It has been found that membranes prepared from certain hydrophilicpolymers containing metals capable of complexing with aliphaticallyunsaturated hydrocarbons, provide high permeability (flux) andselectivity for unsaturated hydrocarbons.

It has been further found that when such polymers have been treated inthe presence of a cross-linking agent under conditions effective tobring about cross-linking of the polymer, the resulting membranesprovide high permeability (flux) and selectivity for unsaturatedhydrocarbons, with improved stability, particularly in the presence ofliquid water.

SUMMARY OF THE INVENTION

The present invention relates to polymer membranes useful for separatingaliphatically unsaturated hydrocarbons from mixtures with saturatedhydrocarbons. More particularly, the present invention relates tomembranes which are prepared from hydrophilic polymers and which containmetals capable of complexing with aliphatically unsaturatedhydrocarbons. Such membranes provide high permeability (flux) andselectivity for unsaturated hydrocarbons.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention concerns membranes prepared fromcertain hydrophilic polymers which contain complexing metals, whichmembranes are useful in the separation of aliphatically unsaturatedhydrocarbons from mixtures containing such hydrocarbons.

In a further aspect of the invention, in the process for preparing themembranes of the present invention, the polymers of such membranes arecontacted with an effective amount of a cross-linking agent underconditions which promote cross-linking of the polymer, thereby resultingin the formation of cross-linked polymer membranes. Such cross-linkedpolymer membranes have been found to provide high permeability (flux)and selectivity for unsaturated hydrocarbons as well as improvedstability in a liquid water environment.

The membranes of the present invention are prepared from at least onehydrophilic polymer which is associated with a complexing metal ion orsalt.

Suitable polymers which may be used to prepare the membranes of thepresent invention include polyvinyl-alcohol, polyvinylacetate,sulfonyl-containing polymers, polyvinylpyrrolidone, polyethylene oxideand polyacrylamide, blends of two or more of these polymers, andcopolymers of the foregoing polymers.

Preferably, the polymers of the present invention are selected such thatthey can undergo cross-linking in the presence of an effective amount ofcross-linking agent under suitable conditions.

Present in the polymer matrix are metal ions or salts which canreversibly complex with aliphatically unsaturated hydrocarbons. It isdesirable that the metal ion or salt be non-reactive with othercomponents of the feed in order to maintain selectivity of the membranefor the unsaturated hydrocarbon, as well as to avoid fouling of themembrane with undesired reaction by-products.

Such metals, which may be used alone or in various combinations, in thepresence or absence of non-metal or non-complexing metal ions, comprisethe transition metals of the Periodic Table having atomic numbers above20. For example, useful metals are those of the first transition serieshaving atomic numbers from 21 to 29, such as chromium, copper,manganese, and the iron group metals, such as nickel and iron. Others ofthe useful complex-forming metals are in the second and third transitionseries, i.e. having atomic numbers from 39 to 47 or 57 to 79, such asmolybdenum, tungsten, and rhenium, as well as mercury. The noble metalssuch as silver, gold and the platinum group, among which are platinum,palladium, rhodium, ruthenium, and osmium, are also suitable.

The preferred metals for use in complexing with olefins are the noblemetals, particularly silver.

The silver metal may be present in the polymer as the ion (preferably inthe +1 state, Ag+), or as a metal salt, such as for example, AgNO.sub.3, Ag(NH.sub. 3).sub. 2⁺, or Ag(pyridine).sub. 2 ⁺.

The amount of complex-forming metal present in the polymer may varyconsiderably, but should be at least sufficient to accomplish thedesired separation. Preferably the metal should be present in an amountsufficient to provide an adequate complexing rate in order to minimizethe membrane surface needed to perform the desired separation.

The membranes of the present invention may be prepared by first forminga "polymer solution" comprising a polymer of the present invention and ametal capable of complexing with aliphatically unsaturated hydrocarbons,in a solvent, which is preferably water.

In a further embodiment of the invention, this "polymer solution" alsocomprises a cross-linking agent in an amount effective to promotecross-linking of the polymer under suitable conditions.

The polymer solution is cast onto a solid support using castingtechniques known to the art, for example, "knife-casting" or"dip-casting".

Knife-casting comprises the process wherein a knife is used to draw apolymer solution across a flat surface to form a thin film of thepolymer solution of uniform thickness, from which the solvent of thepolymer solution is then evaporated to yield the fabricated membrane.When, for example, a glass plate is used as the flat surface, theresulting membrane when removed from the glass comprises thefree-standing polymer. When, alternatively, the flat surface used is anon-selective porous support such as Teflon, Goretex®, Celgard , nylonor polysulfone, the resulting membrane which is formed upon evaporationof the solvent is a composite membrane which comprises the polymer andthe non-selective porous support.

By dip-casting is meant the process wherein a surface of a non-selectiveporous support such as Teflon, Goretex®, Celgard, nylon or polysulfone,is wetted by directly contacting a polymer solution (i.e., without theuse of a knife). Excess solution is permitted to drain from the support,and the solvent of the polymer solution is then evaporated. Thethus-formed composite membrane comprises the polymer and the poroussupport.

In a further embodiment of the present invention, a membrane of thepresent invention is prepared by knife-casting or dip-casting or othertechniques known to the art from a polymer solution which also comprisesa cross-linking agent in an amount effective to promote cross-linking ofthe polymer under suitable conditions. In this embodiment, the membraneafter casting from the polymer solution is treated under conditions oftemperature sufficient to effect cross-linking of the polymer.

Suitable cross-linking agents include formaldehyde, divinyl sulfone,toluene diisocyanate, glyoxal, trimethylol melamine, terephthalaldehyde,epichlorohydrin, vinyl acrylate, and maleic anhydride. Formaldehyde,divinyl sulfone and toluene diisocyanate are preferred.

The cross-linking which occurs using formaldehyde, for example, is acondensation reaction wherein formaldehyde reacts with hydroxyl groupson different polymer chains to form the acetal linkage, --OCH.sub. 2O--,thereby releasing water.

Cross-linking with vinyl sulfone occurs by means of an addition reactionto form an ether linkage between the polymer chains.

Cross-linking with toluene diisocyanate is carried out by placing thepolymer in a solution of the cross-linking agent in a solvent such astoluene, and reacting the toluene diisocyanate with the hydroxyl groupson different polymer chains to form a urethane linkage.

The cross-linking should be carried out at a temperature and for a timewhich are sufficient for cross-linking to occur. It is desirable thatthe conditions under which cross-linking is carried out be selected toavoid accompanying reduction of the hydrocarbon-complexing metal ion.Generally a cross-linking temperature in the range of about 60°-80° C.maintained for about one to five days will be sufficient to effectcross-linking of the polymer. Preferably, the cross-linking temperatureis about 75° C. and the cross-linking time is about three days.

The resulting membranes of the present invention are "solid, homogeneousmembranes" in the sense that they comprise a single polymeric phase inthe substantial absence of a liquid phase.

Advantageously, the membranes of the present invention may be fabricatedof a thinness which is impracticable for the immobilized liquidmembranes previously known to the art, thus providing greatly improvedpermeability with high selectivity for aliphatically unsaturatedhydrocarbons.

The membranes of the present invention may be prepared in thicknesses aslow as 0.1 um, but preferably in the range of at least about 0.5 um, andmost preferably from about 1 um to about 15 um.

The process of the present invention can be employed to separatealiphatically-unsaturated hydrocarbons from hydrocarbon mixtures. Themixture may thus contain one or more paraffins, includingcycloparaffins, mono or polyolefins having different complexing ratesacross the membrane, acetylenes or aromatics. Among the materials whichmay be separated are ethylene, propylene, butenes, butadiene, isoprene,acetylene and the like. The membranes of the present invention areparticularly useful in separating olefins from paraffins. It has beenobserved that conjugated di-olefins permeate the membranes of thepresent invention preferentially over mono-olefins, which in turnpermeate preferentially over paraffins. The membranes of the presentinvention are also useful in separating olefin isomers, such as1-butene, isobutylene, trans-2-butene, and cis-2-butene.

Olefins having from 2 to 20 carbon atoms per molecule, preferably 2 to 8and more preferably 2 to 4 carbon atoms per molecule may be separatedfrom hydrocarbon mixtures containing paraffins, employing a membrane ofthe present invention.

Employing a membrane of the present invention, aliphatically unsaturatedhydrocarbons may be separated from mixtures containing such hydrocarbonsover a broad concentration range, from about 1 weight percent of theunsaturated hydrocarbon to about 99 wt.% mixture thereof.

According to the process of the present invention, an aliphaticallyunsaturated hydrocarbon, such as an olefin, is recovered from ahydrocarbon feedstream by contacting the stream against a first side ofa membrane of the present invention and by withdrawing at a second sideof the membrane a permeate comprising the aliphatically unsaturatedhydrocarbon. The permeate comprises the aliphatically unsaturatedhydrocarbon in increased concentration relative to the feedstream. By"permeate" is meant that portion of the feedstream which is withdrawn atthe second side of the membrane, exclusive of other fluids such as asweep gas or liquid which may be present at the second side of themembrane.

In one embodiment of the process, the aliphatically unsaturatedhydrocarbon of the feedstream is in the vapor phase, and a driving forcefor permeation is maintained by a partial pressure differential acrossthe membrane. The partial pressure differential is preferably within therange of about 0.01 atm (0.15 psi) to 55 atm (809 psi), and morepreferably about 0.5 atm (7.4 psi) to 30 atm (441 psi).

The pressure on the first or feed side of the membrane may range fromabout 0.01 to 55 atm (0.15 to 809 psi).

The pressure on the second or permeate side of the membrane can becontrolled by the action of vacuum, or a sweep gas or liquid. Preferablythe sweep gas or liquid is selected so that it is essentially inert tothe metal ions in the membrane, and is readily removed from thepermeated aliphatically- unsaturated hydrocarbon if necessary.

Examples of suitable sweep gases are nitrogen, carbon dioxide, steam,methane or air.

Examples of sweep liquids suitable for use in the present inventioninclude paraffins with significantly different boiling points than thepermeating species, e.g., hexane, heptane, octane, etc.

In another embodiment, the feed is maintained under conditions oftemperature and pressure such that substantially all of thealiphatically unsaturated hydrocarbon in the feedstream is in the liquidphase, and the unsaturated hydrocarbon is recovered under vacuum in thevapor phase (i.e. pervaporated) at the permeate side of the membrane.

The vacuum on the permeate side of the membrane can range between about1 mm Hg to about 750 mm Hg (0.99 atm or 14.5 psi) at room temperature.

Membrane performance is enhanced by humidification of the feed and/or asweep stream; and this may be accomplished by bubbling the feed and/orsweep stream through water before contacting the membrane, or bydirectly adding water to a liquid feed.

Preferred operating temperatures are from about 0 to about 100° C.

The membrane used in the process of the present invention may beutilized in the form of hollow fibers, tubes, films, sheets, etc. Theprocess is conveniently carried out in a permeation cell which isdivided into compartments by means of a membrane or membranes. Thecompartments will each have means for removing the contents therefrom.The specific design and configuration of the permeation cell will varyaccording to individual requirements of capacity, flow rate, etc.

The process may be carried out continuously or batchwise, in a single ormultiple stages, but preferably in a continuous manner.

The present invention will be better understood by reference to thefollowing examples, which are offered by way of illustration and notlimitation.

In the following examples, flux is expressed in units of kg/m.sup. 2/lday, permeability is expressed in units of cc (STP) cm/(sec cm.sup. 2 cmHg), and the separation factor for the aliphatically unsaturatedhydrocarbon component of the feed is expressed as follows: ##EQU1##where the retentate refers to the mixture on the feed side of themembrane which is rejected by the membrane under given operatingconditions. The flux is determined based on concentration measurementsobtained by gas chromatography, and permeate stream flow ratemeasurements by a flow meter. The relationship between flux andpermeability is as follows:

    Flux =permeability (P.sub. 1 -P.sub. 2)/ l

where P.sub. 1 and P.sub. 2 are the olefin partial pressures in theretentate and permeate streams, respectively, and l is the membranethickness. The partial pressures are determined based on concentrationmeasurements by gas chromatography and total pressure measurements bypressure gauges.

EXAMPLES EXAMPLE 1

To about 44 g of water was added 6 g of polyvinylalcohol with stirringand heating at about 80° C. until a clear solution of the polymer wasobtained. The solution was then cooled to room temperature. To thispolymer solution was added 11.76 cc of 3 M silver nitrate solution (6 gAgNO3) with stirring. The resulting solution comprised about 9.7 wt. %polyvinylalcohol, 9.7 wt. % silver nitrate, and 80.6 wt. % water. Thesolution was then centrifuged for about 5 minutes. Followingcentrifugation a membrane was knife-cast onto a support of microporousteflon (Goretex®). Water was allowed to evaporate from the membrane atambient conditions over a period of about 17 hours. The membrane wasthen heated in an oven at 75° C. for 3 days.

The resulting Membrane A comprised about 50 wt. % polyvinylalcohol and50 wt. % silver nitrate on a microporous teflon support (Goretex®), andhad a thickness of about 12 microns (exclusive of the support).

EXAMPLE 2

To a solution of 5.64 g polyvinylalcohol and 5.15 g silver nitrate inwater was added 3.24 cc aqueous formaldehyde solution (1.2 gformaldehyde) with stirring for 5 minutes. The resulting solutioncomprised about 8.8 wt. % polyvinylalcohol, 8 wt. % silver nitrate, 1.9wt. % formaldehyde and 81.3 wt. % water. The solution was thencentrifuged for about 5 minutes. Following centrifugation a membrane wasknife-cast onto a support of microporous teflon (Goretex®). Water wasallowed to evaporate from the membrane for about 17 hours at ambientconditions. The membrane was then heated in an oven at 75° C. for 3days.

The resulting Membrane B comprised approximately 47 wt. %polyvinylalcohol, 43 wt. % AgNO.sub. 3 and 10 wt. % formaldehyde residueon the microporous teflon support, and had a thickness of about 10microns (exclusive of the support).

EXAMPLES 3

Membrane Samples A and B prepared as described in Examples 1 and 2,respectively, each having a mass transfer surface area of about 63.6cm^(b) 2, were tested for separation factor and permeability employing afeed mixture comprising 1-butene, isobutylene, trans-2-butene andn-butane.

The membrane was placed in a permeation cell comprising a firstcompartment for contacting a feed stream against a first side of themembrane sample, and a second compartment for withdrawing the permeatedfluid from the second side of the membrane.

A vapor phase feed mixture comprising about 20% (by mole) 1-butene, 20%isobutylene, 20 % trans-2-butene, and 40% n-butane under a totalpressure of about 2 atm at about ambient temperature (23° C.) wascontacted against the surface of the membrane sample at a rate of about120 cc/min. The permeate was swept by nitrogen under a pressure of about1 atm and a flow rate of about 100 cc/min. Both the feed and sweepstreams were humidified by bubbling through deionized water prior tocontacting the membrane.

The above process was carried out for a period of 1 day at which timethe separation factor and permeability were determined.

The separation factor and permeability of Membranes A and B for theunsaturated components of the feed obtained after one day's operation ofthe process are shown on the Table below.

                                      TABLE                                       __________________________________________________________________________           Separation Factor with Respect to                                                               Permeability (× 10.sup.-8 cc (STP)                    n-Butane          cm/sec cm.sup.2 cm Hg)                                      1-butene                                                                           isobutylene                                                                         trans-2-butene                                                                       1-butene                                                                           isobutylene                                                                         trans-2-butene                            __________________________________________________________________________    Membrane A                                                                           105  90    60     4    3.3   2.2                                       Membrane B                                                                           110  90    50     4.9  3.7   2.2                                       __________________________________________________________________________

The results indicate that the membranes of the present invention may beemployed for the separation of olefin isomers, such as the buteneisomers. Selectivity toward the butene isomers is as follows, in orderof decreasing selectivity: 1-butene > isobutylene >trans-2-butene.

The separation factor and permeability values of Membranes A and Breported in the Table are similar.

However, while the uncrosslinked membrane, Membrane A, appeared stablein the presence of the humidified feed and sweep streams, this membranedissolves in liquid water. Advantageously, the cross-linked membrane,Membrane B, appears to be stable in liquid water environment.

EXAMPLE 4

Membrane Sample A prepared as described in Example 1 was tested usingthe procedure described in Example 3, except that a vapor phase feedmixture comprising 49 mole % ethylene and 51 mole % ethane under a totalpressure of about 1 atm comprised the feed stream. The separation factorof ethylene with respect to ethane was about 60, and the permeability ofethylene was about 4.7 ×10⁻ 8 cc (STP) cm/(sec cm.sup. 2 cm Hg).

What is claimed is:
 1. Process for separating at least one unsaturatedhydrocarbon from a hydrocarbon feed stream containing said hydrocarbonby the steps of:a. contacting the feed stream against a first side of asolid, homogeneous membrane consisting essentially of a hydrophilicpolymer selected from the group consisting of polyvinylalcohol,polyvinylacetate, sulfonyl containing polymers, polyvinylpyrrolidone,polyethylene oxide, polyacrylamide, copolymers thereof, and blendsthereof, and a metal or metal ion capable of reversibly complexing withthe unsaturated hydrocarbon, said metal or metal ion is distributedhomogeneously in said hydrophilic polymer; and b. withdrawing at asecond side of the membrane a permeate comprising the unsaturatedhydrocarbon in higher concentration than in the feed stream; wherebysaid membrane provides high permeability and selectivity for unsaturatedhydrocarbons and substantially increases the rate at which said permeateis withdrawn.
 2. The process of claim 1 wherein the polymer iscrosslinked.
 3. The process of claim 2 wherein the polymer iscrosslinked by contacting with a crosslinking agent under conditionssuitable to promote crosslinking.
 4. The process of claim 3 wherein thecrosslinking agent is selected from the group consisting offormaldehyde, divinyl sulfone, toluene diisocyanate, and othercross-linking agents including glyoxal, trimethylol malamine,terephthalaldehyde, epichlorohydrin, vinyl acrylate, and maleicanhydride.
 5. The process of claim 1 wherein the metal of metal ion isselected from the transition metals.
 6. The process of claim 1 whereinthe metal or metal ion is silver.
 7. The process of claim 1 wherein thefeed stream is in the vapor state.
 8. The process of claim 1 wherein thesecond side of the membrane is swept by a gas.
 9. The process of claim 8wherein the gas is nitrogen.
 10. The process of claim 1 wherein theunsaturated hydrocarbon is an olefin and the hydrocarbon feedstreamcontains paraffins.
 11. The process of claim 1 wherein the hydrocarbonfeed stream comprises at least one olefin isomer.
 12. The process ofclaim 11 wherein the olefin isomer is selected from 1-butene,isobutylene and 2-butene.
 13. The membrane of claim 1 wherein saidmembrane has a thickness in the range between about 0.1-15 microns.